Pulse control for nonvolatile memory

ABSTRACT

A nonvolatile memory device that uses pulsed control and rest periods to mitigate the formation of defect precursors. A first embodiment uses pulsed bitline control, where the coupling between a memory cell channel and a reference voltage is pulsed when it is desired to change state in the associated memory cell. Each pulse may be chosen to be less than about 20 nanoseconds, while a “rest period” between pulses can be on the order of about a hundred nanoseconds or greater. Because bitline control is used, very short rise times can be enabled, enabling generation of pulse durations of 50 nanoseconds or less. In other embodiments, these methods may also be more generally applied to other conductors (e.g., wordline or substrate well, for program or erase operations); segmented wordlines or bitlines may also be used, to minimize RC loading and enable sufficiently short rise times to make pulses robust.

BACKGROUND

Nonvolatile memory can retain stored information without the continualrequirement of short-interval refresh operations, i.e., such that memorycontents can be retained in the absence of external power for at leastas long as typical on/off times of electronic devices; for thesereasons, nonvolatile memory can save power, enable new applications andfurther extend functionality of portable devices.

To date, one of the most cost-effective forms of nonvolatile memory isflash memory, which unfortunately suffers from use-based degradation.That is to say, flash memory in particular eventually loses its abilityto retain information for any length of time, with current expectedlifetime being about 10,000-100,000 read and write cycles. It isbelieved that one primary cause of this life cycle wear is damage to theoxide layers in these devices from the formation of traps that ariseunder the influence of high electromagnetic fields used in programming(“setting”) and erasing (“resetting”) of device memory cells. Althoughthe physics associated with trap formation are not precisely understood,it is believed that these traps are the result of precursors, which formand develop over short periods of time to ultimately become permanent.Life cycle wear is a significant limitation for nonvolatile memory, andhas generally inhibited application of flash memory in environmentscalling for high read and write turnover (e.g., main memory for portablecomputers).

A need exists for ways of minimizing this damage, so as to extend theuseful life of nonvolatile memory, reduce maintenance, and enable newapplications. The present invention satisfies these needs and providesfurther, related advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of a group of three memory cells105, 107 and 109, arranged as part of a string of memory cells. Whileonly three memory cells are illustrated, it should be understood that atypical string might consist of thirty-two, sixty-four or some othernumber of memory cells. The cells illustrated are nonvolatile memorycells and, in particular, flash memory cells. In conjunction with theexample of FIG. 1, it is desired to charge a floating gate 115 of one ormore memory cells in the string in order to “program” (or “set”) thosecells, or alternatively, to remove charge from the floating gate 115 andthereby “erase” (or “reset”) those cells.

FIG. 2 is a block diagram of a method of pulsed bitline control within amemory cell. The method may be applied, for example, to control the rateat which the floating gates 115 of selective memory cells in FIG. 1 arecharged. By pulsing the current flows and allowing the oxide layer torest before renewed application of voltage, the method of FIG. 2 helpsminimize the formation of precursors, and provides the ability to anneal(i.e., reduce or eliminate) precursors in between pulses.

FIG. 3 is a block diagram of a method of pulsing the flow of currentthrough nonvolatile memory cells. The method may be applied, forexample, to control the charging of the floating gates 115 of selectivememory cells in FIG. 1 during state change operations. Dashed-line boxesat the right side of FIG. 3 illustrate various types of “controlinformation” that determine pulse parameters. In some embodiments,several of these information types may be used and, if desired, thesemay be the subject of dynamic (i.e., run-time) adjustment.

FIG. 4 is a timing diagram of an example of pulsed bitline control. Inparticular, a first signal 403 represents wordline voltage, while asecond signal 405 represents the switching of a substrate channelbetween two voltages (e.g., between “V_(cc)” and ground). The voltageacross a memory cell is indicated by numeral 407, and includes an “on”period defined by numeral 411 and a “rest period” identified by numeral413. A number of pulses “N” is applied as may be desired to ensureproper state change or conditioning operations.

FIG. 5 is a timing diagram that shows defects generated for threedifferent curves, as a function of voltage pulse period. A first curve503 represents a ten volt difference (10VDC) between a floating gate anda substrate channel, while second and third curves 505 and 507 represent9VDC and 8VDC differences, respectively. As indicated in FIG. 5 byreference numerals 509, 511 and 513, depending upon the voltage used,shorter pulse periods generally result in smaller defect generation.

FIG. 6 is a timing diagram similar to FIG. 5, but which representsannealing of defect precursors as a function of the rest period betweensuccessive pulses. As represented by FIG. 6, using an applied voltage of10VDC as a norm, defect generation may be substantially reduced usingthe pulsing scheme represented by FIG. 5 relative to a “conventional,”relatively long, single pulse. As indicated by vertical reference linesand associated range 605, annealing of defect precursors becomessignificant in the rest period range of approximately 100 nanoseconds to2 microseconds, with defect formation substantially disappearing aboveabout 2 microseconds. As indicated by a vertical reference line 607,defect annealing becomes predominant for rest periods of about 800-1000nanoseconds. For higher voltage differences, a longer rest period mayneed to be used, and for voltage differences of less than 10VDC, ashorter rest period may be used.

FIG. 7 is a timing diagram that illustrates the effect of pulsed bitlinecontrol as applied to relative charge of a floating gate.

FIG. 8 is a block diagram of an exemplary flash memory device 801.

FIG. 9 is a block diagram that shows bitline control circuitryassociated with elements of a flash memory device.

FIG. 10 a is a block diagram of a wordline driver 1001.

FIG. 10 b is a timing diagram used to explain a manner of controllingthe circuit of 10 a, so as to program a flash memory cell.

FIG. 10 c shows a bitline driver circuit 1061 as one way of pulsingcurrent across a memory cell element.

FIG. 11 shows an embodiment where bitline control is performed usinglocal segment drivers 1111, to reduce loading effects and therebyimprove pulse rise time.

FIG. 12 shows an embodiment where wordline control is performed usinglocal or segmented drivers 1211 to partition a wordline and therebyreduce loading effects and improve pulse rise time.

FIG. 13 presents a graph of rise time associated with conductor lengthscaling for a 8 GB, 43 nm flash memory device. A first (solid line)curve 1301 represents bitline 2RC rise time, while a second (dashedline) curve 1303 represents wordline 2RC rise time. By scaling conductorlength and applying pulse control in the manner discussed by thisdisclosure, voltage pulses and associated rest periods may be applied toanneal precursors before they generate any substantial oxide layerdamage.

FIG. 14 shows an embodiment where pulse control is applied to substratewell charging.

DETAILED DESCRIPTION

The invention defined by the enumerated claims may be better understoodby referring to the following detailed description, which should be readin conjunction with the accompanying drawings. This description of oneor more particular embodiments, set out below to enable one to build anduse various implementations of the invention or inventions set forth bythe claims, is not intended to limit the enumerated claims, but toexemplify their application to certain methods and devices. Thedescription set out below exemplifies (i) a method of controlling anonvolatile memory cell in a manner that deliberately applies voltagesin the form of multiple pulses, each having short rise time, and (ii) adevice based on such a method, implemented for example as a flash memorydevice. The invention, however, may also be applied to other methods anddevices as well.

I. INTRODUCTION

Oxide layer reliability is of extreme importance in the longevity ofmany semiconductor devices, among them nonvolatile memory cells,capacitors, DRAM cells and other electronic components. In the contextof nonvolatile memory cells, charge may be transferred to and from acharge storage structure (usually a floating gate or charge storagelayer) through an oxide layer, for example, through a tunneling processsuch as Fowler-Nordheim tunneling. Because high electric fields candegrade the oxide layer, the oxide layer can be damaged over time,ultimately affecting the ability of nonvolatile memory cells to “set”and “reset.” Designs presented by this disclosure therefore attempt tomitigate the damage caused by these high electric fields—by spreadingthe transfer of charge through the oxide layer into discrete pulses,with annealing times therebetween, trap degradation can potentially bereduced. The reduction of defect formation under these pulsed statechange conditions is modeled as a two step process. A defect precursormay be formed during the stress condition that requires some time totransform into the final defect structure (e.g., a trap). The defectstructure is stable and difficult to anneal or repair, but the defectprecursors may be removed or annealed if the stress condition is removedfor a periodic “resting” between periods of stress. The retardation ofdefect formation is thus enhanced when the periods of stress are shortand the rest periods are long. That is to say, it is believed thatinterrupting the transfer of charge, if done in a manner to attack theprecursor formation, may help minimize life cycle damage.

It is difficult to generate discrete pulses that are effective for thesepurposes, especially for arrays which are divided into pages of manythousands of cells that are programmed or erased as a unit (e.g., flashmemory cells and SONOS cells). The loading provided by internalconductor routing and transistors used in these designs result in slowpulse rise times, making it difficult to generate short term, discretepulses; that is to say, the rise times for voltage pulses used forprogramming and erasing of these memory types is sufficiently slow thatit is difficult to generate high-voltage, short term pulses that can beeffectively used to retard defect formation during memory cell statechange operations.

Embodiments discussed below are therefore directed to structures thatapply a series of short rise time voltage pulses across the oxide layerof a memory cell. By shortening the interval over which these currentsare applied, ideally to pulses of no more than about 20-50 nanosecondsfor some of today's nonvolatile memory devices, and allowing precursorsto anneal (i.e., to “heal”) before renewed application of the electricfields and associated currents, trap formation can be minimized, therebyenhancing device lifetime. As precursors heal, voltage can then beapplied in one or more discrete pulses to continue transferring chargeuntil a charge storage structure (e.g., floating gate or charge storagelayer) experiences sufficient charge transfer.

In a first implementation, bitline control is employed to apply thesepulses.

In a second implementation, control for a memory array is divided up soas to significantly reduce loading issues. With loading reduced to amanageable level, to enable specific maximum voltage pulse rise times,voltage pulses can be applied to any desired conductor, including to awordline, bit line, substrate well, or other pertinent conductor.Specific segmented driver mechanisms will be discussed which enable thistype of control. In each of these implementations, the total chargetransferred to or from a charge storage device can be the same as isconventional, but with charge transfer broken up as a discrete series ofpulses, separated by a rest period of sufficient duration to mitigatepermanent trap formation.

It is believed that through the embodiments proposed below, memory lifemay be significantly extended, by suppressing defect formationassociated with typical conditioning or memory state change operations.

The embodiments discussed below are primarily couched in terms of flashmemory, NAND flash memory in particular, but the teachings of thisdisclosure may be applied to other forms of memory. For example, anumber of designs for nonvolatile memories have been proposed, includingdesigns based on other technologies. Although known by a wide number ofacronyms and names, these designs usually are based on a material(usually a metal or semiconductor) that reversibly changes a physical orelectrical state under the application of a voltage, field, current, orother effect; the state can be nondestructively sensed, therebypermitting use of the state for information storage. It should bepossible to apply teachings from this disclosure to these othernonvolatile memory forms as well.

II. BITLINE CONTROL

A. Memory Cell Structure and Associated States.

A first embodiment presented by this disclosure provides a method ofcontrolling a bitline of a memory cell in order to limit electric fieldtimes so as to inhibit defect formation; an associated structure is alsoprovided. The design philosophy adopted by embodiments based on bitlinecontrol is that wordline voltage should be held relatively constant(e.g., wordline control separated from application of a multiple pulsesto effectuate a single program cycle) and a bitline should be pulsedbetween ground and another reference voltage to turn off programming (orother voltage activities) and provide a rest state. Because a bitlinepresents reduced loading issues relative to other conductors in a flashmemory device (as the bitline is typically fabricated from a metal,e.g., copper), pulsed operations discussed herein may be effectivelyapplied to substantially shorten pulse rise time. Simply stated, usingbitline control helps facilitate short duration voltage pulses,potentially enabling the generation of pulses of 20-50 nanoseconds orless. The specific design of an array, so as to provide specific risetime criteria associated with bitline voltage pulses, will be discussedfurther below.

FIG. 1 shows a flash memory structure 101 that includes a substrate 103and a group of three memory cells 105, 107 and 109, arranged as part ofa string of memory cells in a NAND flash cell structure. Each string isserved by a common bit line that is coupled to a channel defined by thesubstrate 103. FIG. 1 in fact illustrates a simplified cross-section ofa memory array, meaning that while only three memory cells areillustrated, a typical string might consist of thirty-two, sixty-four orsome other number of memory cells. The cells illustrated in FIG. 1 areadjacent to (i.e., next to) each other within the same string. Eachstring effectively forms a structure that flows current through thesubstrate from a first access transistor 111 to a second accesstransistor 113. The first access transistor 111 effectively couples thebitline to the channel (i.e., to the substrate) through a bitlinecontact 114 to gate the flow of charge into the substrate (that is, thesubstrate channel), while the second access transistor selectivelycouples the channel to a ground connection through point 116, for use inread and verify operations. Each memory cell structure is effectivelyitself also a transistor structure with control gate 117 controllingwhether the transistor is conducting, dependent on the amount of chargestored on floating gates 115 (which sets the effective threshold voltageof the memory cells 105, 107, and 109). To set or reset the memory cell(i.e., to program the cell to a state or erase that state), the cell iscontrolled in a manner that isolates charge on the floating gate, suchthat charge can be retained even in the absence of a power supply.

In the structure depicted in FIG. 1, device state and operation can beexplained with reference to three control points used for operationsinvolving each memory cell. The three control points include (a) thesubstrate (via the two access transistors and a mechanism to selectivelycharge the substrate), (b) the floating gate, and (c) the control gate.To program or erase the cell (i.e., to “set” or “reset” the cell), acurrent is caused to flow to either store charge within the floatinggate or remove charge from the floating gate, with the stored chargeindicating the logic state of the cell; the substrate and the controlgate are used as voltage terminals that can be used to change the stateof the associated memory cell. By applying a voltage difference thatcauses electrons to tunnel from the substrate to the floating gate(e.g., by applying a positive voltage to the control gate with agrounded substrate), electrons are stored on the floating gate tothereby program the memory cell. By applying voltage opposite inpolarity, charge is effectively removed from the floating gate, tothereby erase (or “reset”) the cell.

The illustrated cells are in fact part of a large array consisting ofmany thousands of cells, the cells each organized into “pages” of memorythat are read, programmed and erased together. That is to say, eachcontrol gate 117 is coupled to a “wordline” that extends into and out ofthe page of FIG. 1, where the wordline is coupled to control gates of amemory cell in parallel strings of memory cells (only one of thesestrings is depicted in FIG. 1), usually on the order of a large numbersuch as 32,768 or more strings; this entire slice, or “page” would beread, programmed or erased as a single unit. In addition to thementioned structures, FIG. 1 also illustrates an encapsulation layer121. While a flash structure is illustrated, there are a number of othersimilar structures that use a charge storage element or region in thesame manner as the floating gate described above, for example, asilicon-oxide-nitride-oxide-silicon structure, also called a “SONOS”structure.

Before programming, the second access transistor 113 is turned “off”(i.e., at point “C”). The gate of the first access transistor 111 isconnected to a positive voltage (e.g. “V_(cc)”) so that the channel ofeach string of memory cells will be selectively coupled to an associatedbitline, via bitline contact 114. Each bitline contact will in turn willbe either connected to a positive voltage (e.g. “V_(cc)”) if programmingis to be inhibited on that bitline string or to ground if the memorycells within the string are to be programmed (or “set”). Setting thebitline contact voltage to the same positive value as the accesstransistor gate potential effectively turns off the access transistorand isolates the channel transistors associated with that bitline stringfrom ground. At this time, the wordline representing a page of memorybeing programmed (i.e., “set”), for example, one bit of which isrepresented by cell 109, is raised to a programming voltage. In FIG. 1,this programming voltage would be applied at point “A,” which couplesthe floating gate 115 high and causes charge to flow between thesubstrate and the floating gate (as indicated by arrow 131) and therebycharge the floating gate (if the associated bitline is coupled toground, i.e., at point “B”). For those bitlines that have been“inhibited” or isolated by turning off their access transistor, therising wordline voltage induces a rising channel potential throughcapacitive coupling of the wordlines to the isolated channels. Theincrease in channel potential for inhibited bitlines reduces theelectric fields across the tunneling oxide, and charging of the floatinggates 115 for inhibited bitlines is consequently substantiallysuppressed. The channel potential induced under the inhibited bitlinesis dependent on specific geometries and device designs, but typicallycan rise to 2 to 3 times the inhibited bitline contact potential (e.g.V_(cc)) and charging rates of inhibited cells is typically suppressed bya factor of at least 10⁶. After the wordline voltages have beenincreased and the programming of the grounded bitlines has begun, therate of programming of cells on a selected bitline can also besuppressed by raising the bitline contact potential from ground toV_(cc). The channel potential will rise to V_(cc)-V_(th) and thecharging of the floating gates along this bitline string will besuppressed by a factor of 100 to 1000. This suppression is smaller thanthat experienced on the inhibited bitlines because the channel voltageincrease is not being boosted by a rising voltage on thecapacitively-coupled wordlines, however, the reduction in chargingcurrent is significant enough to provide a “rest” period for the devicebeing programmed. If the bitline contact potential of a resting deviceis returned from V_(cc) to ground, the charging of the selected floatinggates begins again.

In this regard, the gap 131 and a second gap 133 between the floatinggate and substrate can each be modeled as a capacitor—the electric fieldacross these gaps is directly related to the voltage difference at anypoint in time. As a consequence, when a programming pulse is initiallyapplied to the floating gate, with a substrate held at ground, theelectric field is immediately a maximum and begins to decline as thedifference in charge between the floating gate and the control gateequalizes. Large electric fields are observed in the gaps between (a)the control gate 115 and the associated floating gate 117, and (b) thefloating gate 117 and a localized area of the substrate 103, but are ofconcern in the latter gap through which electron tunneling occurs forpurposes of charging and discharging the floating gate. In this regard,the oxide between the floating gate and the control gate (i.e., at thegap 131) is usually thicker and may be made from high dielectricconstant materials to reduce the tunneling to the control gate. It iswithin the second gap 133 that large electric fields can primarily causeproblems that increase leakage and so affect the ability of a memorycell to set and reset (program/erase) in a repeatable manner. Thedescribed behavior of current, electric field and charge is generallydescribed by the Fowler-Nordheim equation for current density,

$\begin{matrix}{{J = {{K_{1} \cdot E^{2}}{\exp\left( {- \frac{K_{2}}{E}} \right)}}},} & (1)\end{matrix}$where “E” is electric field, measured in Volts per unit distance, “J” iscurrent density, and “K₁” and “K₂” are constants. What this equationindicates is that when a charge difference exists between two surfaces,for example, between a control gate and its associated floating gate115/117 or between a floating gate and adjacent substrate 115/103,current generally varies with both the square of the voltage differenceand exponentially as a function of voltage.

The embodiment illustrated in FIG. 1 has the duration of charging ordischarging the floating gate deliberately controlled in duration, andseparated into multiple, discrete pulses, so that the duration of thegenerated electric field is limited and the dielectric between thefloating gate and the substrate or control gate is allowed to “rest,” soas to at least partially anneal or heal defects. That is to say,contrary to conventional wisdom of minimizing program or erase times toemphasize performance, embodiments disclosed herein spread out a programor erase operation into a series of spaced “micropulses,” with asubstantial rest period in between, to emphasize endurance andreliability by retarding defect formation. There are a number of ways toimplement appropriate pulse control, including for those embodimentsthat use bitline control, by pulsing the bitline contact of the firstaccess transistor 111 to effectively modulate the voltage differencebetween the control gate and channel of a memory cell in the process ofbeing programmed. As will be further discussed further below, the risetime associated with these pulses can also be enhanced by minimizing bitline routing and effectively subdividing each bitline into a series ofindividually driven segments (of shorter length).

To program a memory cell using a pulsed bitline, the control gatevoltage for each memory cell may be raised to “V_(pass),” and thecontrol gate for each memory cell in a page undergoing programming maybe further then raised from “V_(pass)” to a relatively higher voltage“V_(prog)” (approximately double “V_(pass)”). During this process, forany memory cell in that page that is to be programmed, the correspondingbitline is then pulsed “on” for a relatively short period (e.g., 20nanoseconds) to connect the corresponding substrate channel to groundfor this period and “off” for a relatively longer period (e.g., 0.1 to2.0 microseconds). During this period, current flow diminishes acrossthe dielectric layer between the floating gate 115 and the control gate117 or substrate 103, respectively, allowing precursor defects toanneal, or partially heal. As will be explained below, depending on thevoltage differences at issue (see FIGS. 5 and 6), a sufficiently shortpulse duration in connection with a sufficiently large rest period maysubstantially suppress the ultimate formation of defects. These pulsesare controlled to still transfer the same amount of cumulative charge toor from a floating gate as a conventional programming pulse (e.g., asingle pulse of a several hundred nanoseconds may be spread out into aten or more repeated individual pulses of less than fifty nanosecondseach) at the expense of increased program or erase time due to “rest”periods intervening between these micropulses.

FIG. 2 presents additional detail on a method of pulsed bitline controlwithin a memory cell, for example, to set or reset the state of thatmemory cell. The method of FIG. 2 may if desired be implemented as partof a single “program pulse” of a multiple-cycle state change operation(known as a “PV” operation). Multiple cycles are frequently used forthese operations, because (a) design constraints are such that multiplecells must be programmed or erased as a unit, and (b) silicon variationsamongst the cells are such that each cell will change state under aslightly greater or lesser impetus. To minimize the likelihood of celldamage, many designs operate by attempting a state change for a group ofcells by applying a relatively small voltage first, to attempt to changestate, and then testing for success of the state change as part of asingle PV cycle. If needed, a slightly large voltage is applied in eachsuccessive PV cycle to those cells which did not correctly change statethe previous iteration, followed by verification, and so on, until everycell in a page or block has the correct state (“set” or “reset”). If atany time, the device reaches a maximum number of cycles or a timeout,the device indicates an error and the block in question may be marked as“bad.” To provide some examples of the Voltages in question, the initialprogramming voltage might be 18 VDC, with pulse magnitude increasing bya half volt each PV cycle up to a maximum of 30 VDC (i.e., twenty-fourcycle maximum). [Notably, the term “program” as used in this contextshould be understood to refer to application of a voltage in a statechange operation irrespective of whether cells are in fact being“programmed” or “erased,” i.e., the use of the term “program” is notlimited to the “setting” of memory cells, but rather, refers to theportion of the PV operation (i.e., the “program portion”) where voltageis applied across the memory cell prior to a later attempt to verifycell contents.] Following the “pulse” portion (or program portion) ofeach PV cycle, affected cells are sensed as part of a verify portion ofthe operation and checked to determine whether they have assumed thedesired state, with additional PV cycles performed as necessary and aspermitted (i.e., up to a maximum number of PV cycles).

FIG. 2 provides a method 201 that may be employed in only a single cyclePV operation or, if desired, to each multiple cycles in a state changeoperation. In the case where it is desired to program memory cells(e.g., a NAND flash), a “page” of cells is processed simultaneouslyuntil every cell in the “page” has the desired contents (i.e., “logic 0”or “logic 1”). In the case of a NAND flash erase operation, an entireblock (i.e., a matrix consisting a series of wordlines and matchingstrings that couple to the same substrate well) is processed until everycell in the block possesses a reset state (i.e., a “logic 1”). Asindicated by numeral 203, a pulse is applied to a memory cell as part ofthe program portion of a single cycle PV operation by first coupling achannel associated with a cell to be programmed to a first voltagereference, for example, to V_(cc). To cause current to flow across apage or block of selected memory cells as desired, the associatedwordline or wordlines are coupled to a second, different voltagereference, (e.g., “V_(pgm)”) as indicated by reference numeral 205.Then, with the wordline or wordlines held at this voltage, the channelsfor each cell to be programmed are pulsed to third voltage reference(e.g., to ground) for a brief period in order to pulse voltage acrossthe cell. Any set of voltages may be used as appropriate to causecurrent to flow across the cell. As indicated by numeral 207, thecoupling of a substrate channel to its associated reference used tocause current to flow across the cell (e.g., to the third reference,such as ground) is then pulsed in order to apply the program portion ofthe cycle. That is to say, the program pulse is effectively applied as aseries of individual control pulses or “micropulses” to couple anddecouple the substrate channel from its reference, and once anappropriate number of these pulses have been applied, the programportion terminates. The memory cell contents may then, if desired, beverified.

FIG. 3 presents a block diagram 301 that provides additional detail onthis operation, implemented as part of the program portion of amultiple-cycle PV operation. In particular, each cycle begins withapplication of the program or pulse portion, during which voltage isapplied across the memory cell in order to transfer charge to or from afloating gate by Fowler-Nordheim tunneling (in the case of a flashmemory cell), as indicated by reference numeral 303. Whereas a single,fixed voltage pulse would conventionally be applied for an intervalsufficient to transfer a desired amount of charge to or from thefloating gate (e.g., a microsecond pulse), step 305 calls foradministering these pulses as a series of discrete pulses(“micropulses”) through bitline control. As indicated by a series ofdashed line boxes 307-317 at the right-hand side of FIG. 3, these pulsesmay be defined by or controlled by any desired criteria. For example,the number of pulses may be specified, per block 307, as may the dutycycle of each pulse, as indicated by numeral 309. If desired, pulsewidth or duration may also be specified, per block 311 (e.g., “20nanoseconds”). In some embodiments, pulse magnitude or other informationmay be provided as part of this “pulse control information.” There alsoexist a number of vehicles for defining this pulse control information.For example, pulse control information may be built into the design of aparticular system or may be made adjustable or programmable at thefactory (for example, by fixing a value, blowing a fuse, or some othermechanism). Still further, in some embodiments, a configuration routinemay be executed during device operation in order to program calibratedvalues into a device register, for use during run-time; if desired, sucha routine may be repeated, or updated for temperature or other sourcesof drift or for other run-time variations. Once the appropriate amountof charge has been transferred, the method then proceeds to the verifyportion of the PV operation, and additional cycles are repeated asnecessary, as indicated by numerals 319 and 321. As mentionedpreviously, for a flash memory embodiment, additional PV cycles mayinvolve progressively higher voltages up to a limit, and may involvesingle or multiple level memory cells.

FIG. 4 presents a timing diagram 401 used to illustrate one possibleoperation of the method of FIG. 4, and shows three voltage waveforms. Afirst waveform 403 represents a logic signal used to gate theapplication of one reference voltage to the wordline, while a secondwaveform 405 represents pulsed bitline control that is used to pulse asubstrate channel between another reference voltage and yet another,third reference voltage. As mentioned previously, the relationshipbetween wordline and channel voltages are such that a first polarity iseffectively applied to set (or “program”) the memory cell, while asecond polarity is effectively applied to reset (or “erase”) the memorycell. A third waveform 407 represents the actual application of voltageacross the memory (i.e., across the floating gate in the case of a NANDflash cell), such that the voltage difference corresponds to a voltageused for programming or erasing; in the context of the embodimentdescribed in FIG. 1, above, the voltage drop across the dielectric willbe such that approximately half of the voltage drop will be between thecontrol gate and the floating gate and the bulk of the remainder of thevoltage drop will be between the control gate and the substrate.Generally speaking, it is generally desirable to have each “on” state ofthe substrate channel be effective to create a voltage drop and, to thiseffect, the wordline may be coupled to the first reference voltage apredetermined amount of time 409 before the corresponding channel for acell to be programmed is brought low. The bitline control is pulsed “on”(i.e., to bring the channel low by connecting it to the third referencevoltage, such as ground) for a duration of time identified by referencenumeral 411, which quickly creates a voltage difference between thewordline and the channel and causes current to flow. The effectivevoltage drop is represented by the third waveform 407, which indicatesthat the voltage difference quickly rises to “V_(prog)” as the bitlineis pulsed “on” (e.g., by turning “on” a transistor associated with oneof point “B” to tie the associated substrate channel to ground) and thenquickly decays to V_(prog)−(V_(cc)−V_(th)) as the substrate iselectrically isolated as the bitline is pulsed “off.” The affectedcell(s) is (are) then allowed to anneal or rest during a rest period413, prior to the application of an ensuing pulse 415, and each repeatedpulse may be applied with a period (or frequency) represented by numeral417. During this period, the wordline voltage continues to be applied,but it is the bitline that is effectively pulsed “off” to allow thevoltage drop across the cell to decay for purposes of annealing. Thepulse duration 411 is selected to be an appropriate duration that, inview of the applied voltage drops, does not promote rapid precursorprogression, and the rest period is selected relative to precursorprogression to be “long enough” so that defects do not arise and becomepermanent during the entire program portion (represented by numeral419). As indicated by the variable “N,” seen at the right-hand side ofFIG. 4, a series of N pulses is applied, where the cumulativetransferred charge would be roughly equivalent to charge conventionallytransferred via a single, uninterrupted pulse.

FIG. 5 presents a timing diagram 501 that shows the effects of pulsevoltage and duration upon defect (e.g., trap) formation, expressed interms of leakage current (SILC, measured in Amps). FIG. 5 shows threecurves 503, 505 and 507, each associated respectively with 10, 9 and8VDC voltage drops, graphed against the logarithm of time. As seen inFIG. 5, the 10VDC curve (numeral 503) demonstrates that leakage currentis relatively small for short pulse periods (less than 0.01microseconds, or equivalently, 10 nanoseconds) but that never quitedisappears. By contrast, the 9VDC curve (numeral 505) demonstratessubstantial reduction in leakage current where program pulses are keptshorter than about 50 nanoseconds in duration, as represented by avertical reference line 509. Finally, the 8VDC curve (numeral 507)demonstrates even greater reduction in leakage current where programpulses are kept shorter than about 200 nanoseconds in duration, asrepresented by reference line 511.

FIG. 5 demonstrates that that for a given voltage difference, defectformation is reduced and substantially eliminated if program pulse timeis kept “short enough” relative to applied voltage drop; it also showsthat better results and greater flexibility may also be obtained byusing relatively smaller voltage differences relative to conventionalpractice. Thus, FIG. 5 demonstrates that, contrary to conventionalwisdom to emphasize high voltages and short programming times,reliability may be enhanced through smaller voltage differences andconstrained voltage pulse times (with longer overall programming times).As mentioned previously, this result is difficult to achieve throughwordline control, and bitline control is effectively applied by theembodiments of FIGS. 1-4 in order to obtain voltage pulses “fast enough”to obtain the benefits represented by the data of FIG. 5.

FIG. 6 presents a timing diagram 601, which is similar to FIG. 5, butplots the effects of variation of the “rest period” (where the bitlineis turned “off”) for discrete pulses of a fixed voltage, e.g., 10VDC. Inparticular, a curve 603 represents the reduction of defects as the “restperiod” becomes longer. The curve demonstrates that for a voltagedifference of 10VDC, “rest periods” of more than 100 nanoseconds (0.10microseconds) begins to yield demonstrable improvement, which continuesthroughout the range 605 of 100 nanoseconds to approximately 2microseconds. Defect reduction becomes substantial for rest periodsgreater than about 500 nanoseconds. As indicated by reference numeral607, defect formation may be reduced by more than 50% for “rest periods”on the order of 1 microsecond or longer (e.g., greater than about 800nanoseconds for the example of 10VDC).

It should be understood that the data represented in FIGS. 5 and 6represent only one dielectric having a specific materials compositionand thickness (e.g., 9 nm Titanium Oxide thickness) and that voltagedifference, duration and rest period may be tailored by one skilled inthe art to have appropriate values, depending on memory device type,materials and effective oxide thickness, and other factors.

FIG. 7 presents another timing diagram 701 that shows pulsed applicationof a voltage difference using bitline control and its effects on overallprogramming time, for a 9 mm Titanium Oxide dielectric. In particular,floating gate charge is illustrated by a staircase pattern 703 and iscontrasted with a curve 705 of current decay pattern associated with asingle voltage pulse. In particular, the staircase pattern 703 isobtained by the use of a pulse period 707 that consists of relativelyshort voltage pulse duration 709 and a relatively long rest period 711,of about 200 nanoseconds and one microsecond, respectively. As wasalluded to earlier, the “rest period” is effectively applied first ineach cycle (e.g., per numeral 713) such that a wordline or other controlsurface may be charged in preparation for a voltage pulse imparted bybitline control. As indicated by numeral 715, a series of “N” steps(i.e., discrete pulses) may be applied until the appropriate amount ofcharge has been transferred (or cumulative current flow has beenobtained); in the example presented by FIG. 7, this charge is seen tocorrespond with a floating gate charge of approximately −9VDC (that is,difference between a substrate and a floating gate in a NAND flashmemory device, expressed as a negative voltage difference since FIG. 7represents charge of the floating gate relative to the control gate). Inthis example, a voltage of approximately 18VDC would have been appliedto a control gate of a NAND flash memory cell, with approximately halfof the ultimate voltage drop obtained between the control gate and thefloating gate, and the remainder of approximately 9VDC dropped betweenthe floating gate and the substrate. As depicted in FIG. 7, whereas asingle pulse might take on the order of 4-5 microseconds tosubstantially charge the floating gate, bitline control with appropriate“rest periods” between pulses may require a longer overall programmingtime (e.g., on the order of between 14-15 microseconds. The relativecharging periods are respectively indicated by numeral 717 (in the caseof the single, indivisible pulse 705) and 719 (in the case of thestaircase pattern 703).

B. Implementation as a Flash Memory Device.

FIG. 8 is a block diagram of a flash memory device 801, used in part toexplain PV cycle operation. In particular, the device includes anonvolatile memory array 803, consisting of a great many individualmemory cells 805. The cells may be configured as single level cells ormulti-level cells. One particular cell, singled out for discussion, isindicated by reference numeral 807. As with all cells in the array, theerasing and programming of the cell 807 is controlled by device logic809, which uses a sense amplifier 811 to detect memory cell values (onepage at a time), and write circuitry 813 to program values into thearray (again, one page at a time). The memory device 801 may beimplemented as a discrete integrated circuit or as a multiple componentdevice.

Programming generally can only occur after all cells that are to bechanged in state have been erased, so typically an entire blockassociated with a page that is to be programmed is first erased; itshould be assumed that this block includes the entire array of memorycells serviced by any one of a small set of word lines and theirassociated strings, e.g., strings of 16 cells serviced by sixteen wordlines, in parallel. In order to perform this erasing operation, thesubstrate well (e.g., via each associated bitline or independently ofthe bitlines) is brought to “V_(erase),” and all associated wordlinesare coupled to ground, so as to reverse bias and effectively dischargeall effected floating gates. As each wordline is grounded the substratewell associated with the block is then strapped to a positive voltage,“V_(erase),” to perform the discharge.

Importantly, as will be described below, pulsed control (to permitdefects to heal) may also be applied to erasing of a memory array—thatis to say, pulsed control may also be applied independent of thebitlines to charge the substrate well (e.g., pulsing of the connectionof the substrate well to “V_(erase)”) or to the wordline (e.g., pulsingof the connection between the wordline to ground) or in another manner.Otherwise stated, to prolong device lifetime, pulsed control may beapplied not only to the programming of cells, but also to the erasing ofcells, any forming or “conditioning” operations applied to the cells, orto other application of voltage.

Irrespective of the erasing methodology, to program a page, a memorycontroller first transfers a page of data to an 10 cache 815 over a databus connection 817, and this data is then transferred into a write dataregister 819. The programming operation is similar to that describedabove for erasing, except that programming occurs for one page at a time(instead of a block) and except that state change is verified bycomparing memory cell contents to an array of desired values (i.e.,values in the write data register 819 that may be “logic 0” or “logic 1”instead of just a universal, single “reset” value). A first PV cycle isapplied to the affected wordline using a minimum voltage for the firstcycle (this voltage for the first PV cycle conventionally may be on theorder of 18VDC). In the manner described in connection with FIG. 1, eachbitline is held low or raised to a nominal voltage (“V_(CC)”) so as topermit transfer of charge from the control gate to a floating gate foreach memory cell that is to be programmed but to inhibit transfer ofcharge for each memory cell that is not to be programmed. For thosecells that are to be “set” (i.e., to have a value programmed into themthat differs from the “logic 1” erased state), the substrate channel iscoupled to ground, such that charge is allowed to build in the floatinggate. Bitline control is applied by pulsing this coupling “on” and “off”in succession in order to pulse the voltage difference across thefloating gate and in order to permit anneal time to the affected memorycell oxide layer. Following the pulse of the program portion of a PVcycle, device logic 809 then performs the cycle's verify portion—thelogic 809 reads the sense amplifier 811 and performs an “exclusive-OR”operation with write data register contents, overwriting write dataregister contents with the result of the “exclusive-OR operation.”Following conclusion of the particular PV cycle, the write data registercontents will indicate those memory cells for a particular page thatmust be again attempted to be programmed i.e., “set,” during asubsequent PV cycle, because they have not yet been properly set. The PVcycle sequence is then repeated as necessary. As mentioned above, thedevice logic 809 alerts the controller when programming is complete orin the event of an error.

FIG. 9 presents block diagram 901 of one implementation of to bitlinecontrol in a flash memory device. In particular, two basic elements areillustrated in FIG. 9, including control logic 903 and a page buffer905; the memory array itself is not illustrated so as to explain some ofthe control functions associated with the device. The control logichandles functions associated with switching, voltage control andassociated timing for the memory array; these functions are illustratedby several logic blocks in FIG. 9. This control logic is genericallyconnected to the device (e.g., to supply a write data register (“WDR”)915, bitline drivers 917 and sense amplifiers 921), though it should beunderstood that individual logic elements depicted (designated bynumerals 907, 909, 911 and 913) may in fact electrically connect to andcontrol only specific elements within the device, e.g., to the memoryarray, sense amplifier, WDR, or only to other logic elements. A statemachine 907 is used to track the success or failure of a current PVcycle and also whether a time-out (excessive number of cycles has beenreached). A mode control and timing block 909 is responsible fordetermining whether a programming or erase operation is in progress and,as appropriate, making decisions as to the particular voltages thatshould be generated and distributed for use by wordlines or bitlines,and associated pulse durations. Pulse duration and associated timingissues, as will be discussed below, may be handled in a centralizedmanner (e.g., common distribution of pulsed voltages) or in adistributed manner (e.g., distribution of a duty cycle signal with localhigh voltage switching for each wordline, channel or well locallyperformed, as appropriate). In at least some embodiments (as will bediscussed below), the use of localized control may be used to helpminimize routing length and thus RC loading, allowing for faster pulserise times. That is to say, by shortening routing length associated withdriving a bitline, rise times may be made “short” relative to thedesired goal of being able to generate programming pulses of less than50 nanoseconds. In at least one embodiment, the routing is designed soas to generate 2RC rise times of less than 16 nanoseconds, and in asecond embodiment, the routing is designed so as to generate 2RC risetimes of no more than 8 nanoseconds. The mode control and timing block909 may also be used to selective enable or disable bitline control (orother pulsing control), to support selective use of a reliability modeor a high performance mode. In a reliability mode, the nonvolatilememory device may be operated with pulse control to apply state changevoltages, to enhance device longevity, in the manner described above. Inthe performance mode, the pulse control can be turned “off,” withconventional voltage application used to minimize the time associatedwith state change operations.

The mode control and timing block 909 also generates write enable andread enable signals 919 and 923. Bitline drivers 917 provide multiplexedcontrol to each of the pertinent bitlines, so that every memory cell ina page being programmed (e.g., corresponding to a selected wordline) maybe individually “set” (i.e., programmed), as appropriate. These bitlinedrivers control the pulsing of voltage references (for each stringundergoing programming). Alternatives for applying bitline controlinclude, among others, pulsing a commonly distributed enable signal,centrally distributing a pulsed voltage reference, and generating pulseslocally within each bitline using appropriate bitline control circuitry.These alternatives are not the only options available to a skilleddesigner. The control logic 903 also includes a wordline select block911 that is responsible for generating an address and enable signal foreach wordline when it is desired to program or read the associatedwordline. A voltage regulation block 913 is also used by the controllogic to generate and distribute voltages needed for device program(set) or erase (reset) operations.

The page buffer 905 typically consists of an internal data cache 906used to hold pages of data being transferred in or out of the memorydevice, the write data register 915 and the sense amplifier circuitry921. When it is desired to write data, a page of data is loaded from thecache into the write data register 915, and the control logic attemptsto program the memory array (not seen in FIG. 9) using the programportion a PV cycle; the write enable signal 919 is asserted for thispurpose. During each verify portion of a PV cycle, the write enablesignal is de-asserted by the control logic and the read enable signal923 is lowered to cause the sense amplifier circuit to check memorycontents and to “exclusive or” memory contents with the contents of thewrite data register, in effect, overwriting the write data register. Thewrite data register will then represent only “differences” betweencurrent memory contents and the desired page of data. Simultaneously,the state machine 907 is updated to reflect whether additional PV cyclesare necessary or whether a maximum number of PV cycles, or a timeout,has been reached. When it is desired to erase (reset) data, an entireblock of memory is processed at a time, and the state machine is usedsimply to identify whether any memory cells in a block have not beenreset to a logic high state.

C. Wordline and Bitline Management.

FIG. 10 a is a block diagram of one embodiment of a wordline driver1001, implemented as a individual circuit 1003 dedicated to eachwordline. In particular the wordline driver includes a decode circuit1005, a discharge/pulldown circuit 1007, and two control transistors1009 and 1011. The decode circuit determines the voltage to be appliedto each wordline and, for this purpose, receives as inputs an enablesignal and wordline address (each respectively denoted by the acronyms“EN” and “ADD”). The discharge/pulldown circuit is used when it isdesired to couple the corresponding wordline to ground (e.g., during anerase operation), and the control transistors 1009 and 1011 arerespectively used when it is desired to couple the wordline to one oftwo voltages, “V_(pass)” or “V_(prog),” respectively. “V_(pass)” is usedas a nominal voltage for each “unselected” wordline during a programmingoperation (i.e., when a different memory cell within the string orbitline is being programmed or “set”). The voltage “V_(prog)” is used toapply a higher voltage to the memory cell that is the subject ofprogramming. Thus, the purpose of the decode circuit for each wordlinedriver is to control the application of the appropriate voltagereference to the associated wordline. In connection with the operationdescribed earlier, “V_(prog)” may be separately defined for each PVcycle by controlling the charge pump (not seen in FIG. 10 a) to generatea desired voltage. In addition, during a read operation, either“V_(pass)” or “V_(prog)” may be scaled by a suitable voltage regulatorto be equal to a threshold voltage, associated with the midpoint betweenvoltages associated with two alternate logic states that are to bediscriminated.

FIG. 10 b is a timing diagram 1031 used to explain a manner ofcontrolling the circuit of FIG. 10 a, so as to program a flash memorycell. In particular, a first waveform 1033 represents the timing of anenable signal, while second and third waveforms 1035 and 1037 representtiming signals used to control the transistors 1009 and 1011 from FIG.10 a. When it is desired to charge a wordline, the enable signal isfirst activated; this activation, a short time later, causes the decodecircuit to turn “on” the first transistor 1009, to apply “V_(pass)” tothe wordline “WL.” The value of “V_(pass)” is selected by the circuitdesigner, to carefully balance the risk of unintended floating gatedischarge against unintended charging of unselected cells, but typicallyis a value of about 9-10VDC. All wordlines of a page where memory cellsare being programmed are thereby raised to “V_(pass).” If it is intendedto program a particular memory cell, then the address value “ADD” isused to cause the decode circuit for the particular wordline to turn“off” the first transistor and immediately turn “on” the secondtransistor 1011, to raise the control gate of the particular memory cellbeing programmed from “V_(pass)” to “V_(prog).” Notably, if the wordlinedriver is being used to control an “unselected” wordline (i.e., one notthe subject of programming), then the second waveform 1035 is held lowfor the duration of the enable signal (as depicted by a dashed line) andthe third waveform 1037 is maintained high, i.e., to not turn “on” thesecond transistor 1011 of the wordline driver, as also indicated by adashed line. Bitline control is effectively represented by a fourthwaveform 1039, which shows pulsed control at a frequency much fasterthan the wordline switching to effectively modulate the voltagedifference existing between the wordline and a matching bitline (for aparticular memory cell). As indicated by dashed lines just above theinitial pulses for the fourth waveform in FIG. 10 b, the pulsed bitlinecontrol may be applied (a) only for programming voltages, (e.g., onlyfor the transition from “V_(pass)” to “V_(prog)”), (b) anytime a voltageis applied to a corresponding wordline (e.g., “V_(pass)” or “V_(prog)”),such as for a state change operation, or (c) only for pass voltages(e.g., for the transition from zero volts to “V_(pass)”). Generallyspeaking, it is expected the best results in terms of device lifetimeenhancement will be obtained where pulsed bitline control is alwaysapplied in conjunction with wordline voltage changes, but designconsiderations for particular designs or applications may cause adesigner to apply bitline control only for select operations; forexample, in the depicted embodiment, pulsed bitline control is appliedonly for programming voltages.

FIG. 10 c shows an embodiment of a bitline driver 1061, which may beimplemented as a tristate device dedicated to each bitline of a memoryarray. This circuit 1061 also includes a bitline pulse control mechanism1065 and bitline-in and bitline enable inputs, used to trigger operationof pulsed control. The bitline pulse control mechanism, in turn,controls two transistors 1067 and 1069. The bitline pulse controlmechanism represents a tristate device in the sense that control overthe two transistors 1067 and 1069 can be separately established, meaningthat each bitline can be turned “off,” and can also be selectivelycoupled to either a selected voltage “V_(CC)” or ground. As wasmentioned earlier, alternatives for pulsed control methods include (a)active control over the enable signal “EN,” (b) use of a dedicated pulsecontrol signal “PC” (either to supply a “live” control signal or toprogram values into registers internal to each decode circuit), and (c)provision of a pulsed supply or ground connection (e.g., “V′_(CC)” and“GND′”). Other alternatives will also be apparent to those havingordinary skill in the art. As indicated in FIG. 10 c, the output “BL”effectively pulses the connection of a substrate channel to a secondvoltage reference so as to establish a voltage pulse across a memorycell, again, with desired pulse magnitude, duration and rest periodcharacteristics, in a manner ideally not subject to detrimental RCloading characteristics.

FIG. 11 shows a schematic where bitline control is used to partition abitline and locally drive individual bitline segments based upon acentrally-distributed power supply 1101. In particular, one or morecounters, clock multipliers or other forms of timing circuitry 1103 areused to derive timing for “on” and “off” timing for bitline control froma base clock 1105. Generated timing pulses 1107 are then fed throughvoltage distribution system 1109 to localized segment drivers 1111. Asindicated in FIG. 11, one or more centrally-generated voltage references(e.g., V_(cc) and ground, as appropriate) may be distributed from acommon power distribution node 1113 to the individual segment drivers1111. Each segment driver also receives a bitline select signal, whichindicates if associated bitlines are to be the subject of programming.Each localized driver may be implemented as simply as a tristate device,controlled in response to the timing pulses 1107. Thus, the localizeddrivers 1111 in this embodiment may pulse the interconnection betweenthis node and the pertinent voltage supply in a manner that minimizes RCloading, i.e., the output of each segment driver 1115 is insulated fromloading effects “upstream” from the pertinent local driver 1111. Becauseloading effects (and slowness of rise time) are RC-proportional, witheach of resistance and capacitance being linear with routing length,halving the length of trace length associated with bitline controlcreates an approximate four-fold improvement in rise time. Each output1115 may then be fed to the associated strings and used as the bitlineenable input seen in FIG. 10 c.

To provide one example of an application of the circuitry of FIG. 11,one recently introduced flash memory device has an 8 GB capacity with abitline 2RC rise time of approximately 266 nanoseconds. This time may betoo long to be able to effectively use bitline control alone to generatevoltage pulses of 50 nanoseconds or less using bitline control. However,by driving bitlines in a segmented manner, for example, by patitioningthe bitline into four “local” segments and using a separate driver foreach, a sixteen-fold decrease in rise time may be achieved (i.e., toapproximately 16-17 nanoseconds), which is short enough to generaterobust pulses of less than 50 nanoseconds. As mentioned elsewhereherein, it is desired for some applications to generate robust pulses of20 nanoseconds duration and generally speaking, these principles callfor a 2RC rise time of no more than approximately 8 nanoseconds, shorterif possible. By further subdividing the bitline, e.g., into six localbitlines, rise time may be further improved, e.g., to 266/(6^2) or toabout 7.4 nanoseconds in this example.

D. Forming Voltages.

The discussion above has primarily focused on pulsed bitline control asa run-time means of enhancing device lifetime. However, certain types ofmemory devices may also be damaged during the application of formingvoltages or other conditioning events to prepare devices for first use.For example, certain types of nonvolatile memory devices must be set orreset for the very first time using a larger-than-typical voltage, toeffectively condition the state change mechanism of the memory cell fora repeatable operation with consistent voltages. Becauselarger-than-normal currents or longer-than-normal currents may flowthrough memory cells during these “conditioning events,” the memorycells are even more susceptible to the formation of undesired defects.The bitline control principles mentioned above may be applied withsimilar effect during the conditioning of memory devices and, in fact,there may be some embodiments where pulsed bitline control is used onlyduring factory conditioning (e.g., because of the use of larger- orlonger-than-normal currents).

III. EXTENSION OF PULSE CONTROL PRINCIPLES TO OTHER CONDUCTORS

The embodiments discussed above rely on bitline control because bitlinesare often implemented via an all metal conductor (e.g., tungsten,copper, etc.) and typically are not encumbered by high loadingassociated with connections to transistor gates. To provide acomparative example, conventional wordline design typically uses apolysilicon material for the wordline, and each wordline in turn iscoupled to literally thousands of transistor gates; in someimplementations, bitlines and associated strings are interleaved,meaning that a device having a 4,096 byte page may effectively havewordlines each connected to 65,536 memory cells (assuming a 2:1interleave ratio, with 8 bits or memory cells per byte). These issuesmake it more difficult to use wordline control to generate robustvoltage pulses, but wordline control is nevertheless possible.

FIG. 12 illustrates an embodiment where wordline control is used, witheach logical page segmented into plural wordlines. For example, whereasone wordline might be conventionally used for two interleaved pages (asin the example presented in the last paragraph), in the example of FIG.12, each page may be implemented using plural wordlines (or equivalentlyarticulated, using a set of wordline drivers to reduce RC loadingexperience in delivering a voltage pulse to each control gate associatedwith a page of memory cells).

A nonvolatile memory array 1201 is seen to include a number ofindividual memory cells arranged in columns 1205 that each represents anindividual page. For example, in an embodiment where page length is4,096 bytes, a column might be associated with 65,536 memory cells asjust described, all managed as a single logical page (as represented bywordline control block 1207). To simplify FIG. 12, only a single page isdesignated by a number, as represented by numeral 1205, but it should beunderstood that a great many pages exist in parallel, as represented byellipses and arrows at the right side of the NV array depicted in FIG.12. Similarly, the matrix of cells represented by this array is accessedby a large number of bitlines, arranged as rows in the array, astypified by a single bitline control block 1209, seen at the right-handside of FIG. 12. A hypothetical 8 GB device might conventionally have arise time of around 1 microsecond. In order to reduce such a rise timeto less than 8 nanoseconds (e.g., to generate a robust voltage pulse of20 nanosecond), the control principles discussed above in the case ofbitline control would call for subdividing the wordline into a smallernumber of locally driven wordlines, each wordline having an effectivelength of “L/n,” where L represents a normalized wordline length and “n”representing the number of divisions (or local drivers needed to reduceloading). Since the loading effects and associated rise time are reducedby a factor of “n^2,” wordline length in this example should be reducedby at least a factor of 12 (6.94 ns rise time). FIG. 12 indicates such adivision, with wordline control being split into a number of localsegment drivers 1211, each driving a segment of substantially smallerlength than represented by page length. Each local driver, in turn, maybe identical to the wordline driver depicted by FIG. 10 a.

An advantage of wordline control is that the same control mechanism canbe used for both programming and erasing operations, e.g., the wordlinecan be pulsed in connection to each of “V_(pgm),” “V_(pass)” and groundto effectively provide precursor annealing for (a) selected pages thatare subject to a state change operation, (b) unselected wordlines thatare in parallel with a page undergoing programming and (c) substratewell charging associated with an erase operation. In this latterexample, the wordline can be pulsed in its connection to ground whilethe substrate well is charged to “V_(erase),” using principles similarto those described above for bitline control.

As should be seen from this example, the application of a series ofvoltage pulses using wordline control is possible, but typicallyrequires a number larger amount of division (and associated drivercircuitry) than achieved through bitline control.

FIG. 13 presents a graph showing 2RC rise time for a 43 nanometer, 8 GBdevice, with the vertical axis representing a logarithmic scale ofnanoseconds and the horizontal axis representing the amount of controlline length subdivision. A first (solid line) curve 1301 representsbitline rise time in this example, whereas a second (dashed-line) curve1303 represents wordline rise time. For example, the value of “1” on thehorizontal axis (representing no wordline or bitline division) indicatesthat for the un-subdivided 43 nm device, wordline rise time is on theorder of one microsecond and bitline rise time is on the order of 266nanoseconds. As indicated by the value of ⅛ on the horizontal axis, bysubdividing the wordline and bitline length each by eight, these risetimes fall to approximately 16 and 7 nanoseconds, respectively.

As alluded to earlier, wordlines and bitlines are not the onlyconductors that can benefit from the teachings presented above, and theteachings presented above may also be otherwise extended to substratewell charging in connection with an erase operation. These principlesare illustrated by FIG. 14, which shows a nonvolatile memory array 1401,and a number of individual memory cells 1403 (only one is numbered tosimplify description). In the example depicted by FIG. 14, each wordlineis seen as controlling a single page 1405 of “j” memory cells; althougha large number of wordlines are present (e.g., thousands), only onewordline shown to simplify description. The wordline of FIG. 14 islonger than the wordline of FIG. 12, assuming conventional design, butthe teachings of FIG. 12 may also be used in combination with theembodiment of FIG. 14 if desired. FIG. 14 also depicts in phantom linesa substrate well 1407, which is associated with a set of “k” wordlinesin parallel, where “k” is string length and the substrate well isassociated “jk” memory cells. Each wordline, in turn, is governed by atime-multiplexed wordline control block 1409, which applies voltage tothe wordline as a unit (e.g., “V_(pgm),” “V_(pass)” or ground, asmentioned above). A single bitline control block 1411 is also indicatedand gates an access transistor for each associated string so as toselectively program a single memory cell associated with theintersection of a particular wordline and a particular string coupled tothe bitline. FIG. 14 presents a method of pulsing a substrate well thatmay be used independent of any bitline control.

It will be recalled from the discussion above that to erase a block ofmemory in NAND flash (e.g., represented those “jk” memory cellsassociated with the substrate well/erase block 1407), the substrate wellis raised to “V_(erase)” while all associated wordlines are coupled toground. In the example of FIG. 14, a substrate well control block 1413may be used to pulse the connection of the substrate well to thisvoltage in order to provide pulsed control to retard precursor formationassociated with an erase operation. That is to say, substrate loadingcan be managed to be even smaller than that used for conventionalbitline design, and through appropriate design (e.g., minimizing tracelength and associated RC coupling associated with substrate powerapplication), an entire substrate well may be pulsed as a unit withappropriately short rise times. The circuit of FIG. 10 c may be used forthis purpose, except that in this case, a well select function would beused in lieu of address and bitline enable functions listed in FIG. 10c.

The use of pulse control during erase operations may also substantiallydeter defect formation by mitigating precursors, allowing them to annealbefore they become permanent. These teachings may be used in lieu of, orin addition to, the teachings presented earlier.

IV. CONCLUSION

A method of programming a nonvolatile memory cell has been described, ina manner that pulses charging to limit the duration of current flows,associated electric fields, and formation of defect precursors. Byapplying programming or erasing in short, repeated pulse intervals, witha sufficient rest period in between pulses, it is believe that thedefects that ultimately degrade nonvolatile memory and shorted devicelifetime may be substantially reduced, in some cases, completelyeliminated. Limiting the magnitude of voltage difference may also helpanneal defect precursors. Voltage pulses of very short duration may beeffectively generating, ideally enabling robust pulses shorter than20-50 nanoseconds in duration, enabling nonvolatile memory devices toeffectively implement precursor annealing.

By using specific circuits as part of the charge mechanisms for thesememory cells, the control gates, substrate, and bitlines for each memorycell may have supplied power precisely regulated to as to reduce thedamage caused by high electric fields. By limiting this damage and thusextending the expected lifetime of nonvolatile devices, it is hoped thatthe potential applications of these devices can be thereby extended,potentially to even eventually include main memory or otherapplications.

Various alternatives to the foregoing techniques will readily occur tothose having skill in the art. For example, as mentioned, the describedtechniques can be implemented in other forms of devices, including otherforms of nonvolatile memory. In addition, it is also possible to designmemory cells so as to effectively apply pulsed control through thesubstrate, the wordlines (i.e., corresponding to a “page” or minimumprogramming unit) or through other conductors without departing from theteachings presented above. Other mechanisms for constraining electricfield generation are also possible, including circuits and techniquesother than the specific pulse control mechanisms presented above.

It should be noted that the various circuits disclosed above may bedescribed using computer aided design tools and expressed (orrepresented), as data or instructions embodied in variousmachine-readable media (i.e., storage media recognizable by computers,such as in electronic memory, CD-ROM, DVD, optical, magnetic or otherformats used to read data by machines). This data may be expressed interms of their behavioral, register transfer, logic component,transistor, layout geometries, or other characteristics. Formats offiles and other objects in which such circuit expressions may beimplemented include, but are not limited to, formats supportingbehavioral languages such as C, Verilog, and HLDL, formats supportingregister level description languages like RTL, and formats supportinggeometry description languages such as GDSII, GDSIII, GDSIV, CIF, MEBES.Other suitable formats and languages may also be used. When receivedwithin a machine such as a computer system, such data or instructionsmay be processed in connection with execution of one or more computerprograms including, without limitation, net-list generation programs,place and route programs and the like, to generate a representation orimage of a physical manifestation of such circuits. The representationor image may thereafter be used in device fabrication, for example, byenabling generation of one or more masks that are used to form variouscomponents of the circuits in a device fabrication process. In oneembodiment, a circuit description is of a type that may be used in thedirect control of integrated circuit forming devices (e.g., in automatedcontrol of fab equipment).

In addition, the description set forth above has also been couched interms of devices that use PV cycles. It is also possible to apply theteachings presented above to devices that do not utilize PV cycles, orthat use a different form of PV cycle operation than the methodologypresented above. Importantly, for those devices that do use PV cycleoperations, a wide variety of controller-memory device interchangeformats exist, including formats where the memory device controls eachPV cycle in a manner transparent to the controller, and formats wherethe controller manages the process and specifically commands each PVcycle, or commands each program and verify portion separately. In oneembodiment, the teachings presented above may be applied to anembodiment where a memory device is compatible with legacy flashcontrollers, i.e., where a controller command for either a multi-cyclePV operation or for individual PV cycles is broken down within a memorydevice into individual voltage steps, ramps, or other power supplycontrol operations, in a manner that is transparent to the controller.If desired, pulse control may be hardwired, calibrated, programmed orotherwise defined or effected. All such operations are contemplated aswithin the scope of this disclosure.

In the foregoing description and in the accompanying drawings, specificterminology and drawing symbols have been set forth to provide athorough understanding of the present invention. In some instances, theterminology and symbols may imply specific details that are not requiredto practice the invention. For example, the interconnection betweencircuit elements or circuit blocks may be shown or described asmulti-conductor or single conductor signal lines. Each of themulti-conductor signal lines may alternatively be single-conductorsignal lines, and each of the single-conductor signal lines mayalternatively be multi-conductor signal lines. Signals and signalingpaths shown or described as being single-ended may also be differential,and vice-versa. Similarly, signals described or depicted as havingactive-high or active-low logic levels may have opposite logic levels inalternative embodiments. As another example, circuits described ordepicted as including metal oxide semiconductor (MOS) transistors mayalternatively be implemented using bipolar technology or any othertechnology in which a signal-controlled current flow may be achieved.The term “coupled” is used herein to express a direct connection as wellas a connection through one or more intervening circuits or structures.The term “exemplary” is used to express an example, not a preference orrequirement. Also, the verb “charge” and reference to “charging”expressed above should be understood to refer to current flow in eitherdirection, i.e., current flow in the FIGS. and discussion may beexpressed in terms of absolute magnitude or current, and may be drivenusing positive or negative voltages or current flows, or both, dependingon implementation.

Accordingly, the foregoing discussion is intended to be illustrativeonly; other designs, uses, alternatives, modifications and improvementswill also occur to those having skill in the art which are nonethelesswithin the spirit and scope of the present disclosure, which is limitedand defined only by the following claims and equivalents thereto.

We claim:
 1. A method of changing state in a nonvolatile memory device,the nonvolatile memory device having plural memory cells, each memorycell having a corresponding wordline, and a charge storage elementoperatively positioned between the wordline and a corresponding channel,a bit of data to be programmed or erased in each memory cell usingmultiple program-verify cycles, the method comprising for each memorycell: coupling the corresponding channel to a first voltage reference;coupling the corresponding wordline to a second voltage reference; andusing a pulsing mechanism to pulse a selected one of (i) the channel or(ii) the wordline to a third voltage reference, to provide for a pulsedvoltage drop across the memory cell; where using a pulsing mechanismincludes repeatedly pulsing the selected one during a program portion ofeach program-verify cycle.
 2. The method of claim 1, where using thepulsing mechanism includes, on a selective basis that is responsive tologic state of a corresponding bitline, pulsing the correspondingchannel between the first voltage reference and the third referencevoltage.
 3. The method of claim 1, where using a pulsing mechanismincludes using a tri-state mechanism under the influence of a pulsedcontrol signal to provide for the pulsed voltage drop.
 4. The method ofclaim 1, further comprising reading pulse control information from aregister that is dynamically programmable, the pulse control informationincluding at least two of: (a) pulse frequency, (b) duty cycle and (c)number of pulses.
 5. The method of claim 4, further comprising readingmode information from the register, including a first setting (i)setting a high reliability mode, in which the pulsing mechanism isselectively turned on, and (ii) a performance mode, in which the pulsingmechanism is turned off.
 6. The method of claim 1, where repeatedlypulsing the selected one includes repeatedly pulsing the correspondingchannel between to the first voltage reference and the third voltagereference during the program portion of each program-verify cycle tothereby apply a plurality of pulses, each pulse having a duration of nogreater than approximately 50 nanoseconds.
 7. The method of claim 6,where each pulse has a duration of no greater than approximately 20nanoseconds, and pulse frequency is no greater than approximately onceevery 800 nanoseconds.
 8. The method of claim 1, where: the channel iscoupled to the third voltage reference via a driver that is undercontrol of a bitline; and using the pulsing mechanism includes pulsingthe driver on and off to sequentially pulse during the program portionof each program-verify cycle an electrical connection between thechannel and the third voltage reference.
 9. The method of claim 8,further comprising assigning multiple drivers to the bitline andsegmenting the bitline into corresponding segments, each segment adaptedto alternatively couple a channel for at least one memory cell to thefirst voltage reference.
 10. The method of claim 9, where at least fourdrivers and corresponding segments are associated with the bitline. 11.The method of claim 1, where: the channel is coupled to an erase voltagereference via a substrate well control mechanism; and using the pulsingmechanism includes pulsing the substrate well control mechanism on andoff to sequentially pulse an electrical connection between the channeland the erase voltage reference during the program portion of eachprogram-verify cycle employed during an erasing operation.
 12. Themethod of claim 1, where: coupling the wordline to the second voltagereference includes coupling the wordline to a nonzero voltage reference;and coupling the channel to the third voltage reference includescoupling the substrate to ground.
 13. The method of claim 1, where:coupling the wordline to the second voltage reference includes couplingthe wordline to a first voltage; and coupling the channel to the firstvoltage reference includes coupling the channel to a second voltage thatis positive relative to the first voltage.
 14. The method of claim 1,where repeatedly pulsing the selected one includes pulsing the wordlinebetween the second voltage reference and the third voltage reference.15. The method of claim 14, where coupling the wordline to a secondvoltage reference includes partitioning the wordline into multiplesegments and assigning multiple wordline drivers per logical page, atleast one driver per segment, and separately driving each the multiplesegments.
 16. The method of claim 15, where at least twelve wordlinedrivers are used per logical page.
 17. A nonvolatile memory device inwhich programming or erasing of a bit of data are each to be performedusing multiple program-verify cycles, the nonvolatile memory devicecomprising: a wordline; a channel; a memory cell between the wordlineand the channel to store the bit of data; a driver to selectively coupleone of the wordline or the channel to one voltage reference; and apulsing mechanism to control an electrical connection between the otherof the wordline or the channel to another voltage reference, in a mannerto apply discrete voltage pulses across the memory cell, where thepulsing mechanism is to apply the discrete voltage pulses across thememory cell repeatedly during the program portion of each of themultiple program-verify cycles.
 18. The device of claim 17, where thepulsing mechanism is controlled by a bitline, to pulse the channelbetween two voltage references during programming operations.
 19. Thedevice of claim 17, where the pulsing mechanism includes a tri-statedevice to control electrical connection between the other of thewordline or the channel and the other voltage reference, and torepeatedly pulse the connection between two different voltage referencesat a frequency that is no less than approximately once per twomicroseconds during a single state change voltage cycle.
 20. The deviceof claim 19, embodied in a flash memory device.
 21. The device of claim17, further comprising a register to store at least two of: (a) pulsefrequency, (b) duty cycle and (c) number of pulses, the pulsingmechanism operative to apply the discrete pulses in dependence upon theat least two.
 22. The device of claim 21, where the register isdynamically programmable.
 23. The device of claim 21, where the registeralso stores a mode setting, including a first setting (i) setting a highreliability mode, in which the pulsing mechanism is selectively turnedon, and (ii) a performance mode, in which the pulsing mechanism isturned off.
 24. The device of claim 17, where the pulsing mechanism andother voltage reference are to apply a rest period between pulsesapplied during the program portion of each of the multipleprogram-verify cycles, each pulse having a duration of no greater thanapproximately 50 nanoseconds.
 25. The device of claim 24, where the restperiod is at least 500 nanoseconds.
 26. The device of claim 25, whereeach pulse has a duration of no greater than approximately 20nanoseconds, and pulse frequency is no greater than approximately onceevery 800 nanoseconds.
 27. The device of claim 17, where the pulsingmechanism is coupled to a wordline, to pulse of the wordline between theone voltage reference and a third voltage reference.
 28. The device ofclaim 17, where the pulsing mechanism is operative to pulse connectionbetween a substrate well and an erase voltage reference during theprogram portion of each of the multiple program-verify cycles during anerasing operation, to erase plural memory cells associated with thesubstrate well as a unit.
 29. A flash memory device, comprising: aplurality of bitlines; a plurality of wordlines; a plurality of flashmemory cells, each cell associated with a unique wordline-bitlinecombination; the bitlines selectively controlling supply of a firstvoltage reference to the memory cells and the wordlines selectivelycoupled to a first voltage reference, to selectively program or erasecorresponding memory cells; and means for repeatedly pulsing during theprogram phase of each cycle of program-verify cycles the connectionbetween at least one of (a) the wordlines or (b) the bitlines, with athird voltage reference, at a frequency, where the frequency is selectedfrom a range between once per five hundred nanoseconds and once per twomicroseconds.
 30. The device of claim 17, further comprising a pluralityof said memory cells, a plurality of wordlines, and a plurality ofbitlines, each memory cell coupled to a unique wordline-bitlinecombination, where: each wordline is selectively coupled to the onevoltage reference; a channel associated with each memory cell isselectively coupled to the other voltage reference under control of acorresponding bitline; and the pulsing mechanism is to pulse at leastone of (a) a selected wordline or (b) a selected channel toward a thirdvoltage reference.